The present invention relates generally to low NOx gas turbine engines, and, more specifically, to a fuel injector therefor.
A gas turbine engine includes a compressor for compressing air which is mixed with fuel and ignited in a combustor for generating hot combustion gases which flow downstream into one or more stages of turbines which extract energy therefrom. An industrial turbine engine is typically used for powering an electrical generator for producing electrical power to a utility grid, and it is desirable to operate the engine with relatively low NOx emissions. A low NOx engine may be operated with steam injection for more effectively achieving low NOx emissions. However, operating a turbine engine dry, or without steam injection, increases the difficulty of achieving suitably low NOx emissions.
Dry low NOx engines require extremely fine control of combustor stoichiometry and very high fuel and air mixing effectiveness. Current engines attempt to achieve these high levels of mixing effectiveness with conventional coannular swirl vane mixers and corresponding fuel injection orifices in which the air and fuel passages require very tight or small dimensional control.
For example, in a conventional fuel injector having coannular swirl vanes, an outer row of swirl vanes is angled circumferentially for swirling the air in one direction, with an inner row of swirl vanes being angled circumferentially in an opposite direction for counter swirling air. Each of the flow passages between circumferentially adjacent ones of the vanes has a throat of minimum flow area which meters the air. And, the fuel is separately metered through corresponding fuel orifices. In order to effect uniform mixing for reducing NOx emissions, the individual vane areas from passage to passage and from fuel injector to fuel injector must be closely matched for correspondingly controlling the fuel-to-air ratio therefrom. Accordingly, the manufacturing process is relatively complex and time consuming to ensure that the vane-to-vane throat areas are within suitably small variations. As engine size decreases, the manufacturing degree of difficulty increases until limited by typical manufacturing dimensional tolerances which prevent further miniaturization for use on small engines.
Furthermore, the individually angled swirl vanes necessarily provide a reduced component of axial velocity since the air is swirled in part circumferentially. In order to provide a sufficient margin of flashback prevention, the axial velocity of the fuel and air mixture discharged from each fuel injector into the combustor should be greater than the conventionally known turbulent flame speed of the fuel and air mixture. Since swirling decreases the axial component of velocity, the swirlers must be made sufficiently larger in size so that the resulting axial component of velocity is greater than the turbulent flame speed.
Yet further, the counterrotating swirling mixtures discharged from the fuel injector into the combustor have a radially varying velocity distribution which affects the combustion process. The discharge velocity is typically low at the centerline of the swirlers and increases radially outwardly. The lower velocity increases undesirable stagnation of the fuel and air mixture, with the fuel injector typically also including a center passage for channeling a portion of the air therethrough for reducing the local stagnation effect.
Accordingly, all of these design factors cooperate together to increase the difficulty of achieving maximum fuel and air mixing with accurate fuel and air metering for promoting low NOx combustion in a gas turbine engine. And, these factors increase the difficulty of achieving low NOx combustion as the size of the fuel injector decreases.